The Latin Empire, established by the Crusaders in Constantinople, had poor control over former Byzantine territory, and Byzantine successor states sprang up in Epirus, Trebizond, and Nicaea. Trebizond had broken away as an independent state a few weeks before the fall of Constantinople. Nicaea, however, was the closest to the Latin Empire and was in the best position to attempt to re-establish the Byzantine Empire.

Theodore Lascaris was not immediately successful, as he was defeated at Poemanenum and Prusa (now Bursa) in 1204, but he was able to capture much of northwestern Anatolia after the Latin Emperor Baldwin I had to defend against invasions from Kaloyan of Bulgaria. Theodore also defeated an army from Trebizond, as well as other minor rivals, leaving him in charge of the most powerful of the successor states. In 1206, Theodore proclaimed himself emperor at Nicaea.

Numerous truces and alliances were formed and broken over the next few years, as the Byzantine successor states, the Latin Empire, the Bulgarians, and the Seljuks of Iconium (whose territory also bordered Nicaea) fought each other. In 1211, at Antioch on the Meander, Theodore defeated a major invasion by the Seljuks, who were backing a bid by Alexios III Angelos to return to power. The losses suffered at Antioch, however, led to a defeat at the hands of the Latin Empire at the Rhyndacus River and the loss of most of Mysia and the Marmara Sea coast in the subsequent Treaty of Nymphaeum. The Nicaeans were compensated for this territorial loss when, in 1212, the death of David Komnenos allowed their annexation of his lands in Paphlagonia.[3]

The accession of Vatatzes was initially challenged by the Laskarids, with the sebastokratores Isaac and Alexios, brothers of Theodore I, seeking the aid of the Latin Empire. Vatatzes prevailed over their combined forces, however, in the Battle of Poimanenon, securing his throne and regaining almost all of the Asian territories held by the Latin Empire in the process.

In 1242, the Mongols invaded Seljuk territory to the east of Nicaea, and although John III was worried they might attack him next, they ended up eliminating the Seljuk threat to Nicaea. In 1245, John allied with the Holy Roman Empire by marrying Constance II of Hohenstaufen, daughter of Frederick II. By 1248, John had defeated the Bulgarians and surrounded the Latin Empire. He continued to take land from the Latins until his death in 1254.

Coin issued by Michael VIII Palaeologus to celebrate the liberation of Constantinople from the Latin army, and the restoration of the Roman/Byzantine Empire.

In 1260, Michael began the assault on Constantinople itself, which his predecessors had been unable to do. He allied with Genoa, and his general Alexios Strategopoulos spent months observing Constantinople in order to plan his attack. In July 1261, as most of the Latin army was fighting elsewhere, Alexius was able to convince the guards to open the gates of the city. Once inside he burned the Venetian quarter (as Venice was an enemy of Genoa, and had been largely responsible for the capture of the city in 1204).

Michael was recognized as emperor a few weeks later, restoring the Byzantine Empire. Achaea was soon recaptured, but Trebizond and Epirus remained independent Byzantine Greek states. The restored empire also faced a new threat from the Ottomans, when they arose to replace the defeated Seljuks.

The Nicaean Empire consisted of Byzantium's most highly populated Greek region, with the exception of Thrace which was under Latin/Bulgar control. As such, the Empire was able to raise a reasonably numerous military force of around 20,000 soldiers at its height – numbers recorded as participating in its numerous wars against the Crusader states.

The Nicaeans continued some aspects of the Komnenian army, but without the resources available to the Komnenian emperors the Nicaean Byzantines could not match the numbers, nor the quality, of the armies that the emperor Manuel and his predecessors had fielded. Western Asia Minor had access to the sea, making it wealthier than most of the splinter states around and in time became the most powerful state in the region, if only for a short period.

Some scholars see the Nicene empire period as an indication of rising ethnic Hellenic consciousness and Greek nationalism. However, these scholars caution that a rise in ethnic consciousness did not have an impact on the official imperial ideology.[4] In the official ideology, the traditional Byzantine view of Byzantium as the successor of Rome was not overturned, as the usage of the word Rhomaioi for subjects of the Nicene emperors demonstrates.[4] The official ideology of the Nicene Empire was one of reconquest and militarism, which was not to be seen in later 14th-century Palaiologan rhetoric.[5] The ideology of 13th-century Nicaea was characterized by belief in the continued significance of Constantinople and the hope to recapture the city, drawing less on claims of political universalism or Hellenic nationalism than on Old Testament ideas of Jewish providence. The emperor in this period is frequently compared to Moses[6] or Zorobabel, or even as the “pillar of fire” that guides God’s people to the Promised Land, e.g. in a speech delivered by Theodore I Laskaris, written by Niketas Choniates.[7]

The rhetoric of this period also glorified war and the reconquest of Constantinople using images not drawn from the Old Testament. For example, in his panegyric of Theodore I Laskarsis, Choniates describes a battle with a Seljuk sultan as a battle between Christianity and Islam, rhetorically comparing the wounds of Theodore, who had himself slain an enemy commander, to those of Christ on the cross.[8] Dimiter Angelov suggests that western crusading ideology may have influenced the development of this view on reconquest, and during this period there is mention that Patriarch Michael IV Autoreianos offered full remission of sins to Nicene troops about to enter battle, a practice almost identical to a western plenary indulgence. However, the granting of such indulgences was short lived, and many of the possible crusader influences seem to have dropped off after 1211.[8]

The Byzantines of the 13th century also drew parallels between the situation of the empire after 1204 and that of Classical Greeks. This evidence has helped to strengthen the view of some scholars, such as A. E. Vacalopoulos, who see these references, combined with a re-evaluation of Byzantium's classical past, to be the genesis of Greek nationalism.[9] With the loss of Constantinople, this comparison played on the idea of "Hellenes" surrounded by barbarians; Choniates equated the Seljuk sultan killed by Theodore I with Xerxes, and patriarch Germanos II recalled the victory of John III Vatatzes as another battle of Marathon or Salamis.[10] In much the same way, Theodore II Laskaris compared his father's victories to those of Alexander the Great and proceeded to extol the martial values of contemporary "Hellenes".[11]

In addition, during this period there seems to have been a shift in how the word "Hellene" was used in Byzantine parlance. Up to this point, "Hellene" had borne a negative connotation and was in particular associated with the remnants of paganism. In this period, however, both the terms "Graikoi" and "Hellenes" appear to enter into the diplomatic usage of the empire as a form of religious and ethnic self-identification, spurred by a desire to differentiate the empire and its citizens from the Latins.[12]Patriarch Germanus II of Constantinople in particular exemplifies this new vision of ethnic and religious identity. His letters equate good birth with the purity of his Hellenistic ancestry, placing more value in his Hellenistic linguistic and ethnic background than in any association with Constantinople, and showing his contempt for the Latins who prided themselves on possessing the city. There is a debate among scholars regarding the exact timing of the shift in meaning of the word Hellene. Roderick Beaton, considering the evidence of the usage of the term "Hellenes" in the 12th century, sees the re-evaluation of the term as occurring before the loss of Constantinople in 1204. In addition, unlike Vacalopoulos,[13] Beaton sees not the birth of Greek nationalism, but rather an embryonic “ethnic” awareness, primarily based around language.[14]

Michael Angold notes that the ideology of the period displays the ability of the Byzantines to react and adapt to changing cultural and political circumstances, including exile, and that the ideological developments of this period were, for the most part, cut short and discarded by the restored empire of the Palaiologoi, as Michael VIII returned to the ideology of earlier periods.[15]

^The Columbia history of the world by John Arthur Garraty, Peter Gay : ”The Greek empire in exile at Nicaea proved too strong to be driven out of Asia Minor, and in Epirus another Greek dynasty defied the intruders.”

^ abA Short history of Greece from early times to 1964 By W. A. Heurtley, H. C. Darby, C. W. Crawley, C. M. Woodhouse page 55 “There in the prosperous city of Nicea, Theodoros Laskaris, the son in law of a former Byzantine Emperor, establish a court that soon become the Small but reviving Greek empire.

^ abAngelov, Dimiter. Imperial ideology and political thought in Byzantium (1204–1330). Cambridge: University Press, 2007. p. 95 Also Kaldellis, Anthony. Hellenism in Byzantium : the transformations of Greek identity and the reception of the classical tradition. Cambridge: University Press, 2007.